Health outcomes in Deaf signing populations: A systematic review

Objectives (i) To identify peer reviewed publications reporting the mental and/or physical health outcomes of Deaf adults who are sign language users and to synthesise evidence; (ii) If data available, to analyse how the health of the adult Deaf population compares to that of the general population; (iii) to evaluate the quality of evidence in the identified publications; (iv) to identify limitations of the current evidence base and suggest directions for future research. Design Systematic review. Data sources Medline, Embase, PsychINFO, and Web of Science. Eligibility criteria for selecting studies The inclusion criteria were Deaf adult populations who used a signed language, all study types, including methods-focused papers which also contain results in relation to health outcomes of Deaf signing populations. Full-text articles, published in peer-review journals were searched up to 13th June 2023, published in English or a signed language such as ASL (American Sign Language). Data extraction Supported by the Rayyan systematic review software, two authors independently reviewed identified publications at each screening stage (primary and secondary). A third reviewer was consulted to settle any disagreements. Comprehensive data extraction included research design, study sample, methodology, findings, and a quality assessment. Results Of the 35 included studies, the majority (25 out of 35) concerned mental health outcomes. The findings from this review highlighted the inequalities in health and mental health outcomes for Deaf signing populations in comparison with the general population, gaps in the range of conditions studied in relation to Deaf people, and the poor quality of available data. Conclusions Population sample definition and consistency of standards of reporting of health outcomes for Deaf people who use sign language should be improved. Further research on health outcomes not previously reported is needed to gain better understanding of Deaf people’s state of health.


Introduction
This systematic review concerns the health outcomes of Deaf people who are sign language users.It identifies and evaluates available information in order to establish the state of our current knowledge as well as future research directions.Globally, WHO estimate that 466 million individuals are living with what WHO define as a "disabling hearing loss" with this figure expected to reach over 700 million by 2050 [1].Of those it is estimated that over 70 million people use one of over 300 sign languages worldwide [2,3].Signed languages are not visual versions of the spoken language of a country or nation, they are separate, fully grammatical living languages in their own right [4].Deaf individuals do not perceive being deaf as a disability, and together form a community, with their own distinct language, culture and history [5].Conventionally the upper case 'Deaf' is used to distinguish them from the greater population of deaf people who are spoken language users and not affiliated with Deaf communities.This distinction between deaf and Deaf is not based on degrees of deafness in an audiological sense, but is rather a sociological distinction, based on cultural-linguistic identity.
Poorer health and mental health outcomes among Deaf communities have been previously observed when compared with the general population [6][7][8][9].Suboptimal management of physical health conditions is also common, posing not just immediate health risk but increasing the risk of long-term complications.A UK study using the EQ-5D-5L recorded a mean health-state value of 0.78 for Deaf people compared to the mean health-state value for the general population of a similar age of 0.84 [10].Common mental health problems have been found to be more prevalent amongst Deaf people in comparison with the hearing population [6].Additionally, Deaf people are more likely to be victims of physical, sexual, and emotional abuse along with neglect, all of which are significant risk factors for poor mental health [11,12].Wide-spread difficulties in accessing health-related information in a signed language, accessing health care in a timely manner, cultural-linguistic barriers in interactions with clinicians and health care providers, and inappropriate diagnostic assessments normed on hearing populations have all been recorded as potential contributors to poorer health outcomes in this population [7,13,14].Poor communication or no access to communication with healthcare professionals in a preferred language can leave Deaf patients with confusion surrounding their condition, or appropriate management techniques, thereby increasing their risk of poor or adverse health outcomes [8].This can lead to late diagnoses, risky patient behaviours resulting from a lack of understanding and information, not having a choice in treatment options, a lack of understanding of health conditions, with adverse consequences for patients.
In summary, the signing Deaf population experience inequalities in health outcomes, and in accessing and navigating healthcare.These inequalities are both widespread and multifactorial, and left unaddressed could be detrimental to the health of the signing Deaf population.A greater awareness and understanding of these issues across the healthcare community is paramount to improving service provision.This, combined with the fact that a comprehensive systematic review of the evidence concerning the physical and mental health of the signing Deaf adult population has yet to be undertaken, highlights the necessity for this review.
There are no similar reviews with a focus on the specificity of health outcomes.PROSPERO records a current systematic review of Inequities Experienced by Deaf and Hard of Hearing Patients in Healthcare Access and Healthcare Delivery [CRD42020161691] and one concerning the prevalence and correlates of mental and neurodevelopmental symptoms and disorders among deaf children and adolescents [CRD42020189403].Neither addresses health and mental health outcomes of Deaf signing adults which is required as a guide to future research and to assist clinicians in their current work.

Research questions
• What does the available literature conclude about the mental and physical health of adult Deaf population(s)?
• How does the health of the Deaf population(s) compare to that of the general population (s)?
• What are the strengths and weaknesses of the available literature?
• What should future research aim to address?

Search strategy
An electronic literature search was used to identify relevant studies up to (13 th June 2023).No starting cut-off date was applied.The following electronic databases were searched using the OVID platform: Medline; Embase; PsychINFO; and Web of Science.The research strategy included the keywords (e.g.deaf*, health*, and sign*) and key terms were truncated and combined through use of the Boolean operators 'AND' and 'OR' (see S1

Study selection
Articles identified by the search underwent a two-step screening process: (i) Title and abstract screening against the inclusion/exclusion criteria in Table 1 below and (ii) Full text screening.Each stage of the screening was completed independently by two reviewers ([AUTHOR ONE] & [AUTHOR TWO]), with a third reviewer ([AUTHOR FIVE]) consulted to settle any disagreements.In the full-text screening, a list of reasons for exclusion were additionally recorded.

Data extraction
Comprehensive data extraction was performed using a pre-specified data extraction tool based on the Cochrane data collection form (Collecting data-form for RCTs and non-RCTs) and additional data extraction criteria to accommodate a range of study designs.This included

Quality assessment
The CASP Cohort Study checklist (https://casp-uk.net/casp-tools-checklists/)was used to assess bias in each study as it was more appropriate to the range of items than other more design-specific checklists in the CASP suite (examples of bias include: was the cohort recruited in an acceptable way?; was the outcome accurately measured to minimise bias?; and how precise are the results?).Two reviewers ([AUTHORS ONE AND TWO]) independently assessed the risk of bias, with quality checks performed on 25% of the extracted papers by a third reviewer ([AUTHOR FIVE]), to ensure consistency.Results were compared with any disagreements resolved by a third reviewer.

Results
The number of records after duplicates were removed was 3,056.Following the screening of titles and abstracts, 2,984 were excluded, leaving 72 full-text articles to be assessed for eligibility.Of those, 37 were excluded.Reasons for exclusion were: wrong outcome (n = 8), wrong population (n = 22), wrong language (n = 1), wrong publication type (n = 5), and records not retrieved (n = 1).A total of 35 have been included in the review (Fig 1).

Study characteristics
Of the 35 included studies, 15 were carried out in the USA, 6 in the UK, 3 in Sweden, 2 in Norway, and 1 each in Australia, Brazil, New Zealand, Napal, France, Thailand, Spain, Denmark, and Austria.Full details of study characteristics of the 35 included articles are shown in Table 2.All 35 items included were peer review journal articles of which 27 reported primary data and 8 were secondary data analyses.29 studies utilised self-report health data.The identified publications varied by research setting and sample size.32 out of the 35 studies took place in economically well resourced, developed countries, 2 from low-and middle-income countries and 1 from least developed countries.Only one study involved a randomised controlled trial.
Of the 35 studies, 26 used comparator populations in their study designs.Of these, 19 used comparisons with the "general population".This was not always distinguished to mean the hearing population.For example, the general population could include people who had a hearing loss but were not signing Deaf people.Comparators were either general population reference data for a particular disease, or general population survey study samples (e.g.Health Survey of England data).Three studies [15][16][17] used external datasets from the general population to construct a comparison group that were to some degree matched for a range of demographic variables (e.g.age and gender).Eight studies sought comparative data between d/Deaf populations.For example, Deaf sign language users versus other deaf people who used spoken language or within Deaf communities whereby Deaf people were distinguished by intersecting characteristics such as ethnicity or sexuality.

Study appraisal
Quality appraisal by study is shown in Table 3. Application of the two initial screening questions in CASP resulted in 28 of 35 studies being eligible for quality appraisal with tool.No studies met all the CASP criteria.Comments on the strengths/weaknesses of studies are incorporated in the presentation of health outcome data below.

Health conditions studied
The summary of each study's findings are reported in Table 4.The coverage of health conditions represented in the included articles is described according to the International Classification of Diseases 11 th Revision (ICD-11) with some studies encompassing more than one of the 26 main categories.15

Health outcomes in comparison with hearing/general populations
Comparison with hearing/general populations were used in 19 studies.The findings of these comparison are presented in the categories by health outcomes.An overview of health outcomes in these studies is reported in Table 6.Cancer.A large-scale study [18] involving secondary data from medical records of Deaf cancer patients found that Deaf people were diagnosed at a more advanced stage of colorectal and prostate cancer (64% of the Deaf group vs 13% of the reference group for prostate cancer, and 100% of the Deaf people were diagnosed at stage III/IV vs 47% for the reference group for colorectal cancer).The Deaf group had larger tumours at the time of diagnosis and their cancers were more likely to have spread to lymph nodes or metastasised to other organs.Deaf people were also more likely to be diagnosed with larger tumours in breast cancers (T2+ size was 60% for Deaf people compared to the reference group 34%) which is related to poorer prognosis, although there was no difference in the metastatic spread between the groups [18].In an age-matched large-scale study, Perrodin-Njoku et al. [17] found that Black Deaf people were more likely to have cancer overall compared to Black hearing people (OR = 3.53, CI 1.61-7.71).
Obesity.Two primary studies [8,9] recruiting 339 and 298 respectively reported significantly higher rates of overweight/obesity among Deaf populations when compared to published data on general populations: 35% of Deaf adults vs 26.6% of adults in the US general population [9] and 72% of Deaf men and 71% of Deaf women were overweight or obese vs 65% of men and 58% of women in the general population in the UK [8] HSE dataset.In the Emond et al. study [8], 90% of the over 65 Deaf group were overweight or obese.Neither a small-scale patient record study (n = 92) [19] nor a case-matched comparator study in Black populations [17] reported any significant differences.However, differences in the mean age of the comparator groups was noted for both studies which could help to explain the non-significant findings.
Cholesterol.Emond et al. [8] found that the mean level for both males and females was 'considerably' lower compared to general population Health Survey of England (HSE) reference data [8] although it was not clear whether the reported difference is statistically significant or not.No potential co-variates were examined.Diabetes.Edmond et al. [8] found that of those Deaf people who reported diabetes, at least half of the participants' diabetes was not under control, which could lead to higher rates of diabetic complications.Perrodin-Njoku et al. [17] reported that Black Deaf people are more likely to have diabetes compared to Black hearing people in the US (OR = 1.77,CI = 1.04-3.02).The type of diabetes was not reported in either study.
Depression/Anxiety.The prevalence of depression / anxiety in Deaf adults was found to be significantly higher compared with the hearing population.A large-scale study (n = 1,704)     [21] reported 33.8% compared with 6.8%; Peñacoba et al. [16] reported mean scores for anxiety of 8.06 compared with 6.60 and for depression 5.01 compared with 3.27 in a case-matched study of Spanish Deaf and hearing adults.Perodddin-Njoku et al. [17] reported no difference in the Black Deaf population compared to the Black hearing population, they stated that the issue of medical mistrust in the general Black community might be a factor in finding no difference between the two groups.
Werngren-Elgstro ¨m et al. [22] in a small Swedish comparative study (n = 45) found that Deaf people aged 65 and over have a higher prevalence of depression compared to their hearing counterparts (37% vs 23%).
Mental well-being.A case-matched study of Spanish Deaf adults (n = 146) reported significantly lower psychological well-being compared to Spanish hearing people: mean score of 24.58 vs 27.44 [16].Rogers et al. [23] small scale validation study reported a non -significant lower well-being mean score on the SWEMWBS (22.82) in comparison with the general population (23.64).
Hypertension.The frequency of raised blood pressure was significantly higher for Deaf people (37%) compared to the HSE data (21%) although the confidence interval was not reported [8].Perrodin-Njoku et al. [17] also reported that Black Deaf people are more likely to experience higher blood pressure compared to the Black hearing population (OR = 1.73).However, a large-scale study (n = 532) by Simons et al. [24] reported that the prevalence for

ICD-11 Code Health outcome Studies
hypertension was significantly lower in the Deaf sample (33%) compared with 46% in hearing sample.Cardiovascular disease.Cardiovascular disease was significantly less self-reported by Deaf people compared to the general population [8].Emond et al. [8] found that treatment rate for Deaf men of all CVD was 45% compared with treatment rate for ischaemic heart disease and stroke of between 61% and 70% for men age aged 55-84 in the general population.
Respiratory / lung conditions.Self-reported Chronic Obstructive Pulmonary Disease (COPD) was less in Deaf adults (1%) compared to the HSE data (4% of men and 5% of women) [8] although the significant difference was not stated.Black Deaf people have a greater likelihood of developing a lung condition compared with Black hearing people (OR = 1.72) [17].Fewer DHH ASL users were reported in emergency department encounters for lower respiratory disease compared to DHH English speakers and hearing English speakers (n = 11 vs n = 62 and n = 29 respectively) [25].Oral health.Vichayanrat et al. [26] reported no differences Deaf and hearing people in prevalence of caries or DMFT (Decayed, Missing or Filled Teeth), and similar oral hygiene status.Those Deaf people who took part in Vichayanrat et al. [26] study were educated at BA and/or Diploma level, therefore, unlikely to be representative of the Deaf population.Arthritis.A self-report study by Perrodin-Njoku et al. [17] found no significant difference in prevalence of arthritis between Black Deaf people and Black hearing people.
Musculo-skeletal symptom.James et al. [25] found that reporting of emergency department encounters for spondylosis, intervertebral disc disorders, and other back problems, was less for DHH ASL users (n = 19) in comparison with DHH English speakers (n = 56) and hearing English speakers.
Pregnancy, childbirth or the puerperium.Of the 32 encounters recorded in emergency department records for other complications of pregnancy James et al. [25], only 3 were DHH ASL users compared with 25 DHH English and 4 hearing English speakers.
Headache.A secondary data study by Wahlqvist et al. [27] reported that Deaf people with USH1 (n = 60) expressed significantly more prevalent problems with headaches compared to the cross section of the Swedish population including those with and without visual difficulties (n = 5738) (40% vs 26% respectively).
Chronic comorbidity.Kushalnagar et al. [20] reported that the hearing sample has more individuals with comorbidities compared to the Deaf sample (40.5% vs 34.2%), although the hearing sample was older than the Deaf sample which could explain the higher prevalence in the hearing sample.
Suicide attempts / suicidal thoughts.The prevalence of suicide attempts in the past year is higher in the Deaf population (2.2%) in the US than observed in the general population (0.4%) [9] and Deaf people reported more suicide attempts in the past year compared with the general population (1.5% vs 0.5%) [28].Deaf people with Usher Syndrome Type 1 (USH1) are more likely to attempt suicide compared with the general population (16% vs 4%) [27].James et al., [25] in a study of emergency department records report no suicide ideation and intentional self-inflicted injury reported for DHH ASL users, in comparison with two were reported for DHH English speakers and three for hearing English speakers [25].
Health status.Using validated assessments in sign language, health status was found to be poorer in the Deaf population compared with the general population in the self-report studies by Rogers et al. [29] (EQ-5D mean index values 0.78 vs 0.84), and Shields et al. [10] (43% vs 17% for depression symptoms).However, Kushalnagar et al. [20], found that hearing people had worse overall health status compared with Deaf people, suggesting that age may be a contributing factor, as the mean age of the hearing sample was significantly older than that of the Deaf adults.The Wahlqvist et al. [27] study reported that the USH1 group have greater problems with fatigue (62% versus 49%), and a loss of confidence (16% versus 6%) compared to the general population.
Quality of life.Fellinger et al. [30] and Henning et al. [31] both reported significantly lower Quality of Life as measured by WHOQOL-BREF in Deaf people compared with general populations.The use of the sign language version of WHOQOL-BREF was not reported in Fellinger et al. [30] study.

Factors identified as influencing health outcomes within the Deaf population
LGBTQ+ status.Kushalnagar et al. [32] found that the Deaf LGBTQ population in the US, in comparison with the Deaf non-LGBTQ population, are more likely to have a personal cancer history (24.1% vs 15.2%), more likely to have a lung condition (23.4% vs 15%), and significantly more likely to experience depression/anxiety (33.3% vs 17.9%).Deaf LGBTQ status was also significantly associated with increased risk for arthritis (RR = 1.26) and for chronic comorbidity (2 or more medical conditions) (RR = 1.25) [32] in comparison with the Deaf non-LGBTQ population.A small-scale study (n = 36) reported that the LGBTQ status were not significantly related to the depression score [33].In a study involving transgender Deaf communities, it was found depression/anxiety was higher for those with nonbinary identities [34].
Educational level.Deaf people with university level education scored higher on psychological well-being compared with other Deaf people [16].In another study, educational levels were found to be significant in explaining psychological well-being score [35].A secondary data study found that Deaf people who reported low educational levels were more likely to be at risk for cardiovascular disease compared with Deaf people with a four-year college degree or more (OR = 5.76) [36].Two small-scale self-report studies [37,38] found that more years of education was significantly associated with higher quality of life for Deaf people.
Employment and economic status.Small-scale self-report study found that Deaf people who are not in employment have significantly lower mental well-being compared to those who are in employment (SWEMWBS BSL mean score 21.10 vs 23.40) [23].Wahlqvist et al. [27] report those with USH1 who are in employment are more likely to attempt suicide compared to the general population who are in employment but those with USH1 who are not in employment the differences in suicidal thoughts are not significant compared to the nonworking group in the general population [15].Income status was reported not to have the presence of cardiovascular disease [36].
Ethnicity.Although some studies include race/ethnicity when describing the study samples, few studies have considered the influence of ethnicity on health outcomes.Perrodin-Njoku et al. [17] identified consistently poor health outcomes for Black Deaf adults with regard to diabetes, hypertension, heart condition, lung disease, and cancer, as well as comorbidity.Anderson et al. [31] reported that individuals who identified as a racial/ethnic minority significantly had slightly higher levels of perinatal depression than those who identified as White non-Hispanic.Kushalnagar et al. [20] reported no significant difference in race/ethnicity on depression/anxiety outcomes.
Gender/sex.Health outcomes by gender/sex were explored in a few studies.Significantly poorer physical well-being outcomes were reported for Deaf females in the validated sign language version study (n = 311) [38].Poorer well-being / quality of life outcomes for Deaf females compared to Deaf males are found [8,16,21,23,30].Kushalnagar et al. [20] higher prevalence of depression/anxiety amongst Deaf females.Deaf men were found to have significantly higher blood pressure (15.9%) compared to Deaf women (7.7%) [8].
Language and communication.Using inadequate access to direct child-caregiver communication in childhood as the independent variable, a large-scale study by Kushalnagar et al. [39] identified that it increased a person's risks of having diabetes by 12%, hypertension by 10%, lung disease by 19% and cardiovascular disease by 61% and increased risk for depression/anxiety by 34% compared to those Deaf people who had adequate access to indirect family communication and inclusion [39].No significant difference in the scores for mood or neurosis were found between those Deaf people who used sign language and deaf people who used spoken language [40].
Family history/personal medical history.Using the sign language version of the assessment and when the validation has been examined, Munro et al. [41] reported that a clinical sample had a significantly lower mean score for wellbeing (18.57;SD = 9.6) compared with a non-clinical sample (27.04,SD = 8.68) on the ORS-Auslan.Overall health status was found to be poorer for Deaf people with depression compared to those with no psychological distress or depression [10].Rogers et al. [42] and Belk et al. [43] found that severity of depression and anxiety was worse for those who self-reported as having mental health difficulties compared to those who did not.
Age.It was reported that diagnosis of depression/anxiety was likely to be young in the large-scale study [20], however the age was reported not to have impact on depression outcomes in Deaf populations in the study by Duarte et al. [38].

Discussion
The findings from this systematic review demonstrate that, in general, physical health and mental health outcomes in Deaf signing populations are worse when compared with general population samples.Additionally, the impact of a health condition on other health outcomes can created further health inequalities.For example, although not a comparison to the general population, Barnett et al. [44] study which involved a whole sample who were overweight/ obese (BMI of 25 or greater) and the biometric outcomes were recorded by a research nurse, 13.5% had diabetes, 37.5% had high blood pressure, 53.8% had high cholesterol, 2.9% had heart 39.6% had a PHQ-9 score indicative of at least mild depression.However, the strength and quality of the evidence available overall is questionable.Firstly, sample definition is poor with inconsistencies in reporting which add to the difficulties in collating and appraising data concerning health outcomes for Deaf adults.The main issues include inaccurate or imprecise descriptions of participants meaning it is hard to discern in some studies who are Deaf sign language users and some study populations incorporated children and young people without any clear distinction from adults in data subsets.Secondly, some studies do not report whether the health outcomes measured used validated standard instruments in sign language nor report potential issues associated with interpreter-mediated assessment and engagement, particularly with regards to self-reported health data.Thirdly, secondary data analysis comparisons with 'general population' data will include some participants who are deaf but not sign language users unless matched 'hearing' samples have been constructed.Fourthly, creating binary comparisons between Deaf sign language users and hearing/non-signing people can cover up issues of diversity and intersectionality within Deaf communities.Where comparison groups are matched on a range of demographic variables, these may still hide different circumstances associated with variables e.g.social determinants that are more prevalent amongst Deaf populations e.g.under-employment or direct discrimination.
Furthermore, gaps remain in the knowledge of specific health outcomes as there is no reported health outcome data for the Deaf population in 11 out of the 26 (42.3%) of the ICD-11 disease classification categories, including, for example, diseases of the immune system, visual system and nervous system which indicates clear deficits in health outcome data for this population.The bias towards studies concerning mental health might be in part explained by the longstanding development of specialist mental health services for deaf people in some countries such as the UK and US garnering funding for evidence-based practices.The major neglect of data on physical health outcomes might be related to the considerable difficulties in recording and extracting routine health data that is specific enough to differentiate Deaf people from anyone who is categorised with a hearing disability in routine health data collection [45].For example James et al. [25] in a study on emergency department encounters, highlighted the possibility that DHH ASL users were being mis-recorded as DHH English speakers.The invisibility of the Deaf population within clinical records is likely to contribute to a lack of focus on whether their outcomes are similar to those of the bigger population of adults with a hearing loss or disability but who are not members of a cultural-linguistic minority whose engagement with health services is fundamentally mediated by problems of linguistic access and cultural competence [46].In addition, the overwhelming majority of the included studies concern Deaf people who reside in economically well-resourced countries.Yet, nearly 80% of people who experience deafness, whether sign language users or not, reside in low-and middle-income countries [1].
It is also notable that some study designs are not present in this field which would enhance knowledge.For example, there is a lack of qualitive, exploratory designs specific to health outcomes, there is very little longitudinal study data (whether retrospective or prospective) that might start to reveal patterns of health inequalities experienced by individuals over time.Although there is some large-scale secondary data analysis based on routinely collected national data, most disease-specific studies rely on small population samples the biases of which are not clearly examined.There is also a lack of information on inclusive research designs that involves Deaf populations at all stages of the research process.
The reasons for the health inequalities experienced by Deaf individuals are multiple and complex, both access to and communication with health services and clinicians are commonly cited problems [7].Around 5% of deaf children have one or more parents who are d/Deaf, meaning that the vast majority are born to hearing parents, who usually have little experience of deafness and often have little or no knowledge of signed languages [47].Age-appropriate literacy remains a key barrier to accessing information for a great many d/Deaf people and is especially apparent amongst sign language users of previous generations whose access to and quality of education has been particularly poor [48,49].Socio-economic inequalities are a common experience faced by Deaf people, for example, Deaf people are more likely to be unemployed and those who are in employment are likely to experience inequalities in earnings [50].These issues of inequalities in language development, educational attainment, employment, and income would have impact across this group's lifetime relating to the likely disposable resources Deaf people have such as quality of life and opportunities.Prilleltensky et al. [51] has argued that without opportunities to access rights and equality, a person would be unable to fulfil their well-being potential, and that "social justice can help manage social determinants of health in a more equitable manner" [52, p.8].Despite the legal rights to promote equality and combat discrimination in some countries (e.g. the Equality Act 2010 in the UK, and Americans with Disabilities Act in the USA) and the legal status of sign languages in some countries, Deaf people are likely to be faced with discrimination whether be it direct or indirect.The responsiveness of health services and health interventions to provide and promote understanding of health conditions in a signed language is also identified as inadequate in many countries.Deaf individuals are up to 7 times more likely to experience poor health literacy than their hearing counterparts, something which is closely tied to unhealthy behaviours, limited healthcare seeking behaviours, decreased service use and poorer health outcomes [53][54][55].Studies show that Deaf people have limited knowledge of the symptoms of certain medical emergencies, such as heart attacks and strokes, and that in the US, only 61% would contact the emergency services in such cases [56].Research has also explored the issue of inadequate adaptation of clinical and psychological assessment tools for use with Deaf patients [57,58], resulting in both under and overdiagnosis of potentially serious health conditions and inadequate tracking of recovery [43,59].Understandably, Deaf populations have previously reported feelings of mistrust towards healthcare professionals [7], these populations are also found to be less likely to see the value in healthcare consultations when compared with the general population [60].Aggravating this, many Deaf patients also have difficulty complaining about the healthcare barriers they face, as complaints processes often do not accommodate for sign language users [61].Consequentially, healthcare professionals are unaware of the relevant issues, and no action is taken to amend them.

Conclusion
This comprehensive systematic review on health outcomes in Deaf signing populations has highlighted health inequalities in comparison to general populations and within their own communities.This review has highlighted the need for better classification in routine health records, better data on different kinds of health inequality, and better coverage of diseases to understand the Deaf experience and a wider range of study designs yielding different forms of evidence.Without understanding Deaf people's experience, it would be challenging to improve their healthcare and health outcomes.Addressing health inequalities in practice and research requires the inclusion of Deaf people's priorities for better health.
Table for full details).

Table 1 . Inclusion and exclusion criteria.
study samples, methodology, limitations, evidence gaps, results, and a quality assessment for critical appraisal.Three reviewers ([AUTHORS ONE, TWO, and THREE]) extracted data independently.Results were then compared and discussed, with any disagreements settled between them and an additional reviewer ([AUTHOR FIVE]).
* Signing Deaf populations are defined using a sociological/cultural-linguistic definition, referring to a particular group, not defined by the audiological condition of not hearing, but rather those who use a signed language as their first or preferred language-and share a culture.There is not one global population but signing Deaf populations exist in each country.
out of the 26 classifications are encompassed within the 35 included studies.The greatest number of studies (25 out of 35) concern ICD-11 Code 6 Mental, Behavioural or Neurodevelopmental disorders.This includes anxiety/depression, mental wellbeing, psychiatric disorder, mental distress, and schizophrenia (See Table 5 for reported health outcomes by ICD-11 main classification code.).The remaining 11 classifications where there are no outcome studies identified are: 01 Certain infectious or parasitic; 03 Diseases of the blood or blood-forming organs; 04 Diseases of the immune system; 08 Diseases of the nervous system; 09 Diseases of the visual system; 10 Diseases of the ear or mastoid process; 14 Diseases of the skin; 17 Conditions related to sexual health; 19 Certain conditions originating in the perinatal period; 25 Codes for special purposes; and 26 Supplementary Chapter Traditional Medicine Conditions-Module I.

Table 4 .
CT = Can't tell, N = no.CASP criterion 7 reports the results and is excluded here as they are included in the main text.In the CASP criterion 8 (precise) Y indicates Confidence Intervals (CIs) were reported, and N that they were not.Study outcomes. https://doi.org/10.1371/journal.pone.0298479.t003

Table 4 .
(Continued) byKushalnagar et al. [20]reported 24.9% compared with 21.7% and that it occurred at an earlier age; Kvam et al.' Norwegian population study

Table 4 .
(Continued) Alzheimer's and related dementias, ASL-American sign language, BRFSS-The Behavioural Risk Factor Surveillance System, BSI-Brief Symptom Inventory, CCI-Charlson Comorbidity Index, CDC-Centre for Disease Control and Prevention, CORE-OM-Clinical Outcomes in Routine Evaluation-Outcome Measure, DASS-21-Depression Anxiety Stress Scale-21, DHH-Deaf and hard of hearing, DHS-Deaf Health Survey, ELCEs-Deaf Profile Early Life Communication

Table 6 . Overview of health outcomes in comparison with hearing / general populations. Health outcome Signing Deaf populations in comparisons with hearing/general population samples
[17]el et al., 2018) a more advanced stage(Druel et al., 2018)[18].•Higherrisk of cancer overall for Black Deaf people when compared with the general Black population (Perrodin-Njoku et al., 2022)[17].